As is now well known, in MRI, a body to be imaged (such as a human body) is placed in a static magnetic field such that, for a selected region of the body, the polarities of certain nuclei in the body tend to align in the direction of the magnetic field. By imposing a gradient magnetic field across the body and then applying an RF field at the Larmor frequency, selected nuclei can be tipped over from their magnetically aligned position into a position 90 or 180 degrees off of alignment. Removing the RF transmission signal causes the selected nuclei to realign. During the realignment, the nuclei generate RF signals that can be detected by an RF receiving coil. The resulting RF signals received by the RF receiving coil are processed to derive a cross-sectional image of a portion of the body such as a head, spine, etc.
In MRI devices, several apparatus and methods have been devised to create both the static magnetic field and the RF field. The static magnetic field, for example, has been created using both tubular coils, (where the body is passed inside a tube), and transverse magnets, (where the body is positioned between opposing pancake-like magnets). The transverse magnet arrangements have been commonly referred to by the type of structure that supports the transverse magnets, such as "C", "H", and "Four Post" arrangements. These transverse magnet arrangements are beneficial over tubular coil arrangement in that they provide improved access to the body during the imaging procedure. This can be advantageous, for example, when the person being imaged needs to be physically accessible to receive medical treatment during the MRI process.
The RF field must also be created during the MRI process and is commonly done so by a set of coils disposed within the MRI device. One such arrangement provides saddle-shaped coils or solenoids arranged around the body being imaged. The saddle-shaped coils or solenoids produce RF fields in manners well known in the art. These arrangements, however, are problematic since they restrict the access to the body being imaged. This deficiency is especially exemplified in transverse magnet systems where the open access that is gained by the transverse magnets (for the static field) would be at least partially lost by the presence of the saddle-shaped coil or solenoid (for the RF field) that may be close to and around the body being imaged.
An RF antenna that provides improved access to the body is disclosed in Akgun, U.S. Pat. No. 4,968,937. Akgun discloses an RF antenna in the form of two sheets of conductive strips that are arranged on opposite sides of the body being imaged. Additional sheets, providing a return current path for the conducting strips, are arranged in composite with the conducting strip sheets. In operation, Akgun describes the current passing through the strips as causing the sheets to act as transmission lines that are tuned to a desired resonance frequency by a tuning network.
While the Akgun arrangement provides improved accessibility to the patient during the MRI procedure, it suffers from problems with complexity that are associated with the current conducting strips on the composite sheets. The composite of conductive sheets with strips and return paths is believed to disadvantageously increase manufacturing cost. In addition, homogeneity of the RF field may be reduced if current carried through the respective conductive strips is not carefully controlled.
Another RF transmitter arrangement is described in McCarten, et al., commonly assigned U.S. application Ser. No. 08/025,418. McCarten discloses an RF coil formed of flat conductor lengths that make serially-connected, oppositely directed turns. The conductor lengths are arranged near the static field magnets on opposite sides of the patient. In particular, the conductor lengths are formed in annular depressions created by the magnetic shims associated with the static field magnets. In this arrangement, the RF coils do not add any extra obstacle to physically accessing the patient during the MRI procedure.
Due to the configuration and location of the conductor lengths in the McCarten RF coil, the McCarten device, however, requires considerable RF power at the transmitter in order to tip the selected magnetically aligned nuclei in the body.
An RF transmitter that provides good access to the patient, operates at lower power levels is desirable.